Chalk line, i.e. a carpenter's tool used for making a
straight line on a log or piece of wood before chainsawing
or cutting it.
Instead of white chalk, black battery carbon is used. The
carbon is moistened and rubbed on a nylon line attached
lengthwise to the wood. Then the line is pulled up tight and
quickly released with a snapping motion. The carbon on the
line leaves a straight, black line on the wood.
baktaw